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Friday, 29 May 2015

Know your Solar Batteries

Lead-acid
batteries are commonly used as an energy storage device for the stand-alone or
off-grid solar PV system. Batteries are rated according to their voltage,
Ampere-hour (Ah) storage capacity and their ability to deliver the stored
energy over a given period of time called the C-rating. Figure 1 shows a 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

Fig. 1: 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

When a voltage, higher
than the battery, is applied to the battery through an inverter or a charge
controller of the solar PV system, a charging current will flow into the
battery and will charge it. The flow of charging current is opposite to the
flow of load current supplied by the battery. The rate of charge or current
that will flow into the battery will depend on the difference between the
battery voltage and the applied voltage (the voltage supplied by the solar PV
system etc.).

Once
the battery is fully charged, it is necessary to stop the charging otherwise it
will damage the battery. Its here the charge controller comes into picture. A charge controller, usually used with the solar PV
system, is thus essential to ensure that the battery is never overcharged.

Overall efficiency of charging and discharging of LA battery:

The
temperature of operation and the rate of charge/discharge affect the
performance of the battery. Because of the electrical resistance (internal
resistance of the battery and the electrical path), some of the electrical power (I2R) and hence energy supposed to
charge the battery is converted into heat. During the charging process, some of
the hydrogen ions combine with free electrons and are converted into gaseous
hydrogen. When hydrogen is lost during charging, energy is also lost.

Thus, the
charging efficiency is about 95%.

Similarly, energy is also lost during
discharging of battery and hence,

the
overall efficiency of charging and discharging of a lead-acid battery is nearly
90%.

Since
the battery loss due to resistance is proportional to the square of the
current, therefore

high current charging or discharging will result in higher
internal losses and reduced overall efficiency.

Effect of Temperature on Battery performance:There is also an increase in
the battery temperature during charging and discharging, particularly at high
rates, due to higher I2R
losses. If the battery remains too warm for a longer duration its life is
reduced. Thus, charging and discharging rates should not exceed the specified
values given by the manufacturer for a given type of battery. Increase in
temperature accelerates the chemical reaction process in the battery. Thus, in
high temperature the battery is liable to get self-discharged without any external
load and hence has to be kept in a cool place with preferable temperature of 27
oC.

A
fully charged 24 V battery will have a voltage of around 25.5 V. As the
electrical load, connected to the battery through the inverter, draws the
current, the level of charge in the battery is reduced and thus the voltage of
the battery. The battery voltage will fall down quickly at first, but as the
discharging continues, the rate of voltage drop slows down and will reach 24 V
when the battery has drained to half of its rated charge capacity. As the
battery approaches the fully discharged state, the voltage starts to fall at a
greater pace again. When
the electrical load is drawing a heavy current from the battery, the voltage
will drop. The larger the battery, smaller is the voltage drop.

What is different in a Deep discharge flooded lead-acid battery?

Deep
discharge flooded lead-acid battery, commonly used with solar PV system in
India, uses antimony to strengthen the lead and can be used down to 20% of
their initial capacity. The plates are thicker, with less area and hence are
designed for sustained lower level currents. Flooded batteries need water
topping at least twice a year. Although deep discharge batteries are capable of
deep discharge, but their life depends on the depth of discharge incurred every
time during the operation. Deep discharge i.e. 70 to 80% discharge of the rated
charge leads to decrease in battery life.

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About Me

Hello, I am Manoj Nair, specialized in Heavy Electrical Equipment (M-Tech). Also Life Member of ISTE and Associate Member of IE(India).

Currently I am working as Consultant in the field of Electrical Distribution and Energy Conservation. Also worked as Faculty in Electrical Engg, engaged in teaching post graduate and under graduate students of Electrical & Electronics engineering. I have authored/co-authored 8 books on various subjects of electrical engineering.

Also worked in Electric Utility as an engineer and handling the various jobs related to installation and maintenance of distribution lines, transformers and the metering equipments.

In the free time I switch over to blogging, discussions related to power system, developing MATLAB coding for problems related to power system.